The present invention relates to a display device, a display control apparatus for controlling the display thereof, and an information processing apparatus including the display control apparatus.
Generally, an information processing system (or apparatus) uses a display device as a means for realizing an information visual expression function. As is well known, CRT display devices are widely used as this display device.
In display control in a CRT display device, a write operation for writing an image to be displayed into a video memory (to be referred to as a VRAM hereinafter) provided in an information processing apparatus and a read operation for reading out display data from the VRAM are independently executed.
In the above CRT display control, write access of display data to the VRAM to update display information and read access for displaying an image are independently performed. This results in an advantage in that programs of an information processing system can write desired display data at an arbitrary timing without taking account of a display timing.
Generally, however, the depth of CRT display devices increases in proportion to the display area, and consequently the volume of a whole CRT display increases more and more. That is, CRT display devices are unpreferable in respect of miniaturization because the degrees of freedom of, e.g., installation site and portability are impaired.
A liquid crystal display (to be referred to as an LCD hereinafter) is available as a display device for compensating for this drawback. This is so because the ratio of the thickness to the display area of an LCD is much smaller than that of a CRT. An example of LCDs having this property is a display (to be referred to as an FLCD hereinafter) which uses a liquid crystal cell of ferroelectric liquid crystal. One characteristic feature of the FLCD is that the liquid crystal cell holds a display state even after the end of application of an electric field. That is because the liquid crystal cell of the FLCD is sufficiently thin, and so long and narrow FLC elements in the cell maintain their respective oriented states even after the electric field is removed. The FLCD using the FLC elements with this bistability therefore has characteristics of storing the display contents. The details of the FLC and the FLCD are described in, e.g., Japanese Patent Application No. 62-76357.
In driving of the FLCD, the FLCD keeps displaying images by storing the display images, unlike CRTs or other liquid crystal displays, so a certain time margin is produced with respect to a continuous refresh driving period. As a result, in addition to this continuous refresh driving, so-called partial rewrite driving is possible by which the display state is updated only in those portions where display contents are changed.
In this manner, display is performed by the partial rewrite, i.e., by transferring only a portion in which the display contents are altered to the FLCD. Accordingly, the FLCD is required to have intelligence to a certain degree in order to receive and display the transferred image.
Also, the display speed of the FLCD slightly changes in accordance with the temperature (the higher the temperature, the higher the display speed). Therefore, it is desirable that the data transfer period change in accordance with the temperature of the FLCD. Assume, for example, that the FLCD is used as a display of an information processing apparatus such as a personal computer, and that only a portion in which the display contents are altered is transferred to the FLCD with the information processing apparatus previously switched on. In this case, if the FLCD is switched on at that moment, only the transferred partial image is displayed, i.e., an overall image cannot be displayed.
That is, normal images cannot be displayed if the information processing apparatus one-sidedly transfers display image data to the FLCD. Accordingly, some communications must be performed bidirectionally.
On the other hand, the faster the transfer of display image data to the FLCD, the better the transfer. Unfortunately, bidirectional communications through a bus unavoidably sacrifice the transfer rate of display image data.
Also, an image display device displays image information (including character image information) supplied from an image supply device such as a host computer. Such an image display device is usually so designed that image adjustment, e.g., contrast adjustment and brightness adjustment, can be performed in real time in accordance with the display contents or the external environment, such as an illumination state, by manipulating a slide switch or a dial switch.
Two methods are available as the method of performing this image adjustment. In the first method, an input means, such as a slide switch or a dial switch, for inputting an image adjustment instruction signal is provided in an image display device. On the basis of the input image adjustment instruction signal from this input means, the image display device changes an image display parameter. In the second method, this input means for inputting the image adjustment instruction signal is provided in an image supply device such as a host computer. On the basis of the input image adjustment instruction signal from the input means, the image supply device changes an image processing parameter for producing image information to be supplied to the image display device.
Unfortunately, in the first method, it is impossible to perform fine image processing (image adjustment) because the image display device singly changes the image display parameter.
In the second method, on the other hand, fine image processing can be performed by the image supply device. However, if the image supply device and the image display device are installed apart from each other, it is difficult for a user to input an image adjustment instruction signal while monitoring the display screen. This makes smooth image adjustment impossible.